A solid electrolytic capacitor is composed of a porous material of valve-action metal such as tantalum, niobium and aluminum to be used as a first electrode (anode), an oxide film formed thereon to be used as a dielectric, and a solid electrolyte formed thereon to be used as part of a second electrode (cathode) The solid electrolyte serves to electrically connect between the entire surface inside the porous material and an electrode lead. Therefore, it needs desirably to have such a high conductivity that can reduce the resistance of the capacitor itself. On the other hand, the solid electrolyte also needs to serve to recover an electrical short-circuit due to a defect in the dielectric film. Thus, metals having a high conductivity but not having the dielectric recovering function cannot be used as the solid electrolyte. Manganese dioxide, TCNQ complex etc., which can change into an insulator by heating due to the short-circuit, have been used. In particular, manganese dioxide, which has thermal stability at 240.degree. C. or higher, has been generally used because the capacitor is subject to a temperature of 240 to 260.degree. C. when it is mounted on a printed board.
Thus, a material used as a solid electrolyte for solid electrolytic capacitor needs to satisfy the three requirements, i.e., a high conductivity, a dielectric recovering function and a thermal stability at 240.degree. C. or higher.
Manganese dioxide, which has been conventionally used as the solid electrolyte, has sufficient properties as to the dielectric recovering function and thermal stability. However, its conductivity (about 0.1 S/cm) is not always sufficient for a solid electrolyte for solid electrolytic capacitor. Therefore, in recent years, a solid electrolytic capacitor using a conducting polymer, such as polypyrrole, polythiophene and polyaniline, to satisfy the three requirements of the solid electrolyte as a solid electrolyte has been developed. A solid electrolytic capacitor using polypyrrole is already commercially available.
In general, there are four problems in solid electrolytic capacitors using a conducting polymer. First, the conducting polymer has to be surely formed on the entire surface inside the porous material. Second, in a high-temperature atmosphere to which the solid electrolytic capacitor is exposed, the conductivity of the conducting polymer may not be reduced. Third, the conducting polymer has to be formed on the oxide film, with such a film thickness that can stand a stress generated in the expansion/contraction of covering resin. Fourth, to reduce the manufacturing cost of the capacitor, the conducting polymer layer needs to be simply formed.
To solve the above problems, a method of preparing a solid electrolyte by polymerizing a thiophene derivative with iron (II) compound has been proposed (Japanese patent application laid-open No. 2-15611 (1990) and U.S. Pat. No. 4,910,645). This method is suitable for solving the second problem because the polymer of the thiophene derivative has a thermal stability higher than the polymer of a pyrrole derivative.
Also, the inventors of this invention have proposed a method of conducting the chemical oxidative polymerization by using a dopant with a large molecular size, and an oxidizing agent, such as a copper (II) compound, a silver compound etc. (Japanese patent application No. 8-185831 (1996)).
However, in the method disclosed in Japanese patent application laid-open No. 2-15611 (1990) and U.S. Pat. No. 4,910,645, there is a problem that the conductivity is reduced due to the occurrence of de-doping in a severe high temperature atmosphere at 125.degree. C. or 150.degree. C. The reason is as follows: In forming the conducting polymer layer by using a high-concentration oxidizing agent, it is formed near the outer surface of the capacitor element and thereby it becomes difficult for a solution used to form the conducting polymer to permeate into the central part of the capacitor element. Therefore, the thickness of the conducting polymer layer at the central part of the capacitor element is extremely reduced. As a result, due to the heating in the heat test, the thinned inner conducting polymer layer must be changed into an insulator.